1. Field of the Invention
The invention relates in general to the non-destructive pulsed laser marking of objects in a pattern defined by a digital micro-mirror device. The laser energy induces a color change in a radiation sensitive material that is contained in the object without damaging the object.
2. Description of the Prior Art
It is well recognized that ultraviolet and visible light lasers are suited to marking objects by reason of causing color changing reactions in a radiation sensitive material that is included within an object. The radiation sensitive material strongly absorbs the laser energy and undergoes a color change. Except for the energy absorbing material the object preferably absorbs very little of the laser energy. Infrared lasers generally tend to damage the objects because the energy is adsorbed and heats the object. Generally, infrared lasers are not used for non-destructive marking purposes. See, for example, Mercx et al., U.S. Pat. No. 6,214,916, and Faber et al. U.S. Pat. No. 5,489,639.
It is well recognized that pulsed UV lasers find application in the marking of titanium dioxide containing substrates. See, for example, Murokh U.S. Pat. No. 6,429,889 (consumable articles). See also, U.S. Pat. Nos. 5,501,827, 5,091,284, 5,415,939, 5,697,390, 5,111,523, 4,595,647, 4,753,863, 4,769,310, 5,030,551, 5,206,280, 5,773,494, and 5,798,037. Laser marking in the ultraviolet region causes a color change, typically, by photochemical reaction. It is customary to use masks of one description or another between the laser and the substrate to be marked. The mask serves to define the pattern of the coherent UV light that impinges upon the substrate, and, thus, the image that is recorded on the substrate. Alternatively, controlled beam deflection produces images one dot at a time, roughly comparable to a conventional dot matrix printer. See, for example, Faber et al. U.S. Pat. No. 5,489,639. Typically, the titanium dioxide in the substrate is white, and it turns black when coherent UV energy of at least a minimum flux density impinges on it in the pattern defined by the mask.
The use of pulsed laser energy to mark ceramics and glasses that contain radiation sensitive inorganic pigments is known. See, for example, Gugger et al. U.S. Pat. No. 4,769,310.
Pulsed lasers deliver very short but powerful bursts of energy. The duration of a typical pulse is from approximately 5 to 100 nanoseconds at as much as several megawatts of power. Many substances degrade at high levels of coherent UV or visible flux density if they absorb any significant amount of the coherent energy. Typically, titanium dioxide is present in a substrate material that is substantially UV transparent and does not absorb any significant amount of the UV energy. Titanium dioxide absorbs UV energy and undergoes a photochemical reaction so that it changes color from white to black. It is thus possible to mark titanium dioxide containing substrates with coherent UV energy without degrading the substrate to any visible degree. Other substrates are designed to absorb UV energy so as to prevent its reflection from the absorbing substrate. The titanium dioxide in the UV transparent substrate changes color at a level of coherent UV flux density that is at or above the level at which the typical UV absorbing substrate degrades significantly. The use of pulsed coherent UV energy at a controlled flux density combined with titanium dioxide in a visible part of the object permits objects to be marked without causing visible physical degradation to the object. See particularly, Murokh U.S. Pat. No. 6,429,889. Where the marking is made visible by reason of the physical degradation of the object (as by ablation, melting or burning) high levels of flux density are employed, the coherent marking energy is generally supplied in the visible or infrared regions, and the substrate that suffers ablation absorbs the coherent energy.
The energy absorbing characteristics of natural and synthetic silicon and organic plastic materials are well known and need not be repeated here. Where coherent ultraviolet energy is employed to generate the desired marking, the substrate material from which the object to be marked is made should be selected so that does not absorb enough ultraviolet energy to cause ablation, thermochemical reaction, melting, vaporization, or other visible degradation.
Conventional laser marking systems generate the desired marking pattern using masks, linear marking, or dot matrix methods. The linear marking and dot matrix methods require careful coordination between the movement of the object to be marked and the laser beam. If the mask is moving so as to generate different patterns, the same careful coordination is required.
Digital micro-mirror devices (DMD) are well known. Typically, a digital micro-mirror device consists of an array of tiny mirrors (typically, several million per square inch), wherein the angular position of each mirror element is individually controllable between at least two positions that are angularly off from one another by approximately 10 to 20 degrees. A mirror base is located behind the mirror elements. The individually addressable mirror elements are tiltably mounted on mechanical hinges, and typically the array of mirror elements overlays a layer of controlling circuitry in the mirror base, all of which is mounted on a semiconductor chip. The mirror face of a DMD is composed of a generally rectangular grid array of the tiny rectangular mirror elements. A typical mirror element is about 16 micrometers square, and the individual elements are separated from one another by a distance of about 1 micron. Because of these separations, a portion of any energy that falls on the mirror face will bypass the mirror elements and fall on the mirror base. Individually controlled tilting of the mirror elements in the array around at least one axis allows energy that is reflected from the mirror face to be formed into a predetermined pattern. Further, the mirror face can be substantially instantaneously reconfigured responsive to digital signals to form a different pattern. Such reconfiguration generally requires approximately 25 microseconds. Digital micro-mirror devices have been proposed for use in high-resolution projectors. Proposals have been made to utilize these characteristics of a digital micro-mirror device in printing using generally continuous, visible, and non-coherent light. See, for example, Florence et al. U.S. Pat. No. 5,461,411, and Allen et al. U.S. Pat. No. 6,414,706. It has also been proposed to use a DMD to define a pattern of ultraviolet light on a substrate to catalyze a chemical reaction on the substrate in the pattern formed by the light. See Garner U.S. Pat. No. 6,295,153.
There are spaces between the adjacent edges of the individual mirror elements in the mirror array on the mirror face of a DMD so as to allow them the freedom to tilt independently responsive to commands by the control circuitry. Radiant energy that bypasses the individual mirror elements impinges on the base, including the controlling circuitry, hinges and supporting substrate below the mirror face. This bypass radiant energy should be absorbed, reflected away from the target substrate, or conducted elsewhere so that its random reflection does not blur the intended image that is reflected from the mirror face to the intended target. Absorption of the bypass energy causes an undesired build up of heat in the base. Also, particularly with coherent UV energy, the structure and circuitry below the mirror face tends to be damaged or disrupted by high levels of absorbed bypass radiant energy. There is a maximum acceptable level of absorbed bypass energy flux that can be tolerated by a DMD. Above this level, the DMD is at significant risk of failure.
The maximum level of coherent energy flux density that a DMD can tolerate is generally substantially below the minimum level of coherent energy flux density that is required to cause titanium dioxide or other radiation sensitive marking materials to change color. The level of flux density that is required to ablatively mark a substrate is generally several orders of magnitude greater than that required to cause radiation sensitive material in the target to change color.
The level of flux density of coherent energy is conventionally adjusted by expanding or condensing a beam of such energy to achieve a desired level of flux density. See, for example, Gatrner U.S. Pat. No. 6,295,153. Typical applications entail either expanding or contracting a beam of energy, but not both. There are practical limits to how much a beam of energy can be expanded and contracted. Uniformity of flux density across the cross-sectional area of the beam degrades with excessive expansion and contraction.
These and other difficulties of the prior art have been overcome according to the present invention.
According to the present invention, an object substrate that contains a markingly effective amount of a radiation sensitive material, for example, titanium dioxide, in a visible portion thereof is subjected to a patterned pulsed beam of coherent energy, for example, UV laser energy, having a level of flux density that is at least sufficient to cause the radiation sensitive material to change color, but which is insufficient to cause visible physical degradation of the object substrate. The radiation sensitive material, the wavelength of the coherent energy, and the substrate material of the object are selected so that the radiation sensitive material strongly absorbs the coherent energy, and the substrate of the object does not. The pulsed beam of coherent energy derives its pattern from the configuration of individual mirrors on the face of a DMD as the energy is reflected from that mirror face. The pattern caused by the positioning of the individual mirror elements is instantaneously reconfigurable (within approximately 25 microseconds) responsive to digital signals received by the DMD. With pulsed coherent energy the mirrors are reconfigurable within the period between the pulses when the laser is not illuminated. The level of flux intensity at the mirror face is less than the level at which the DMD is at risk of damage or disruption. The level of flux intensity at the object substrate to be marked is sufficient to cause the titanium dioxide or other radiation sensitive material to change color, but below the level at which the object substrate is visibly degraded, and substantially above the level at which the DMD is at substantial risk of damage or disruption. To accommodate these inconsistent requirements, the cross-section or footprint of a typical pulsed beam of coherent energy is expanded before and condensed after impinging on the mirror face of the DMD. Better markings appear to be achieved if the beam of energy is generated with such a flux density that it requires expanding to protect the DMD from damage. Also, most suitable lasers produce pulses of energy that are above the threshold that typical DMDs can tolerate. Typically, pulses of coherent energy are serially generated, for example, by a laser, and each pulse possesses a level of flux density that is substantially above the level at which the DMD is at substantial risk of damage or disruption. Controlling the marking operation according to the present invention permits a DMD to be used to accomplish the marking of objects with substantially instantaneously variable patterns. Some wavelengths of energy are more degrading or disruptive to digital micro-mirror devices than others, depending upon the nature of the material from which the DMD is constructed and its configuration. Most such micro-mirror devices do not function well at temperatures above approximately 60 to 70 degrees centigrade. Infrared wavelengths generally quickly overheat the DMD. Visible wavelengths are more likely to cause overheating than UV wavelengths. With particularly sensitive marking materials the flux density of the visible light can be kept below that at which overheating occurs while still achieving a good mark. It is very difficult to protect the DMD from damage when the wavelength of the coherent energy is in the infrared range. The preferred wavelengths for the coherent light are in the UV range. The practical limits of expanding and contracting of the cross-sectional area or footprint of a pulsed beam of coherent UV energy are not exceeded where the marking occurs due to a change in the color of titanium dioxide or other radiation sensitive material in a visible part of the object substrate. By selecting the object substrate so that it absorbs little or no coherent energy relative to that absorbed by the marking material that is included within the substrate, it is possible to avoid visibly damaging the object substrate. The pigment particle size and loading rate are preferably optimized to achieve the desired detectable marking at the minimum level of coherent flux density. The DMD can be further protected if at least part of the bypass energy that falls between the individual mirror elements and onto the mirror base can be reflected or conducted away from the base. If this is not possible, then the flux density of the energy that impinges on the DMD must be kept below the level at which all of the bypass energy can be absorbed by the DMD without substantial risk of damage.